1. Field of the Invention
The invention relates to an electric radiant heating element for heating a plate, particularly a glass ceramic plate.
2. Prior Art
Such radiant heating elements conventionally have a temperature sensor, whose outer tube is made from an insulating material, particularly quartz or quartz glass. It is either incorporated into the expansion system of the sensor, in that it forms a tube with a low expansion coefficient in which is located a tension member with a higher expansion coefficient, or it is mounted on an expansion tube. It is necessary in order to ensure the necessary air gaps and/or creepage distances in the space between the heating resistors and the plate through which the temperature sensor passes. It must be borne in mind that glass ceramic plates are electrically conductive at the working temperature, so thaty the necessary insulating gaps must be maintained here.
These quartz glass tubes are not only relatively expensive, but are also fragile and require special measures when they are fitted to the radiant heating element.
SUMMARY OF THE INVENTION
An object of the present invention is consequently to provide an electric radiant heating element, where there is no need to individually insulate the temperature sensor.
This object is inventively achieved by at least one spacer for maintaining a minimum spacing between the insulator or the heating resistors arranged thereon and the temperatures sensor or plate.
This ensures a minimum air gap between the sensor tube, which is preferably metallic and optionally also grounded, and the heating resistors. Tests have revealed that the most significant risk of a possibly dropping below a possibly predetermined air gap, results from the fact that in continuous operation the insulator, which can be in two-layer form, is curved upwards and therefore brings the heating resistors closer to the sensor tube. This is reliably avoided by the spacer. Simultaneously the relatively stiff sensor tube also ensures that the insulator remains flat. The spacers need not constantly engage on the temperature sensor. There can instead by a certain spacing if it is ensured that the desired minimum spacing is still provided in the case of engagement of the spacer. This minimum spacing can e.g. be 3 mm, whereas the total air gaps, whilst incorporating the temperature sensor spacing from the plate, should be approximately 8 mm.
Preferably the spacer is formed by at least one projection of the insulator, which is in particular located in an upheated, central region of the radiant heating element and can be shaped in one piece with the insulator material, e.g. during the vacuum forming thereof. Particular preference is given to a construction with two spaced projections provided in the central region.
In addition to or in place of the projection engaging on the temperature sensor, it is possible to provide a sensor for engaging on the underside of the plate. In this case it keeps constant the spacing between the plate and the insulator, so that the latter cannot "grow" towards the temperature sensor. A combination of both measures is also conceivable, in that e.g. a projection emanating from the insulator has a contact or bearing surface for the plate and a support for the temperature sensor. This can be realized in the form of a staggered cam or a projection with a hole for the temperature sensor.
According to another embodiment on the sensor is mounted a spacer, e.g. in the form of a circular or rectangular disk, which is provided for engaging on the insulator and/or on the plate. As a result of an asymmetrical construction of this plate, it is also possible to set different spacings with respect to the underside of the glass ceramic plate and the insulator.
As a result of their sensitive quartz tubes, the hitherto known temperature sensors have had to be very flexibly arranged and were preferably placed in top-open slots of an otherwise all-round edge of the insulator and a support tray surrounding the same and in said recesses had a clearance so that no stresses could occur. However, this led to a heat bridge to the exterior, so that not only was energy lost, but also there was unnecessary heating of the hob support. Through the possibility of using a rigid metallic sensor, it is now possible to substantially rigidly mount the temperature sensor and therefore the complete regulator, in that it is e.g. inserted with its usually somewhat tapered free end into a support tray hole which is closed at the top, whilst the switch side can be fixed by a holding angle bracket to the support tray of the radiant heating element. The insulating rims or edges can also be closed towards the top and therefore provide a more complete insulation. Therefore the temperature sensor can be inserted in relatively narrow holes in the edge and essentially forms a seal with respect to the outside.
Further advantages and features of the invention can be gathered from the subclaims, description and drawings, whereby the individual features can be realized singly or in the form of subcombination in any embodiment of the invention, as well as in similar or related fields, e.g. in connection with the heating of baking ovens or the like.